In conventional motorcycles, the aerodynamic action produced by the air stream striking the moving vehicle results in a system of forces applied to its barycentre or centre of gravity G as outlined in FIG. 1 of the accompanying drawings.
Said system of forces consists essentially in a drag R, parallel to the ground, in a lift Z, usually quite a low one equal to about 10% of the drag, and in a pitching moment M, usually a diving one, in consequence of the conditions of movements in the field of air around the motorcycle. In fact, in so far as a conventional motorcycle is concerned, the median line of the air stream appears as a camber LM, indicated by the dash-dot lines in FIG. 1, the fore part of its concavity being turned upwards, the rear part downwards, as a result of the distribution of the pressures on to the ground ahead of and behind the motorcycle, said pressures being positive ahead of and negative to the rear of the vehicle, according to the short-dashed line prin FIG. 1. In the following specification, the median stream line is considered as an infinitesimal flux tube, capable of producing the same forces as in practice are produced by the movements in the whole field of moving air surrounding the vehicle, i.e., as if the entire capacity of the jet striking the motor vehicle passed through said flux tube. The field of motion, as perturbed by the motorcycle, extends to infinity; yet in practice at a distance from it of the order of its linear dimensions said perturbations are negligible; accordingly, the dimensions of said jet will be considered as twice or three times greater than the linear dimensions of the motorcycle. Since the system of the barycentrical forces considered above resolves into one force only, it is necessary to vertically displace the resistance R by one length .DELTA. h = R/M,
so that said resistance or drag will in consequence be applied at a height h' from the ground where H' = h .+-. .DELTA. h, the negative sign being effective if the moment M is a diving one.
Given that a and b are the respective longitudinal distances between the center of gravity G and the axles of the front wheel and the rear wheel, and p is the wheelbase, the load variations in consequence of aerodynamic effects will be as follows:
On the front wheel: ##EQU1## On the rear wheel: ##EQU2## Since h' is in general positive and of the order of 1/3p, a is of the order of 1/2p and R much greater than Z (R.gtoreq. 10Z), .DELTA. Z.sub.R will be negative and .DELTA.Z.sub.A positive.
Namely, in a conventional motorcycle, in consequence of the aerodynamic effects, the rear wheel is overloaded while on the contrary the front wheel is lightened, according to about .DELTA.Z.sub.A.congruent. 0.4 R and .DELTA.Z.sub.R .congruent.-0.3 R.
This represents a serious inconvenience, since, as is well known, the drag R grows in quadratic relationship to speed according to the following expression: EQU R = C.sub.x S 1/2.rho.V.sup.2,
where the product C.sub.x S can be 0.24 in the case of an unstreamlined motorcycle and 0.21 in the case of a streamlined motorcycle.
Accordingly, there exists a critical speed: ##EQU3## at which the load Z.sub.A on the front wheel is overcompensated by the aerodynamic effects and in consequence the motorcycle will tend to turn over backwards, pivoting on the line of contact between the rear wheel and the ground.
Considering as an instance Z.sub.A = 100 kg, such a limiting condition takes place at speeds ranging from 450 to 500 km/h. Nevertheless, in the case of riding speeds ranging from 1/3 to 2/3 V.sub.R, typically attained during competition, a substantial lightening of the front wheel takes place sufficient to seriously compromise the directional stability of the motorcycle.